Python and ctypes: how to correctly pass “pointer-to-pointer” into DLL?

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Post actual code. The C/C++ code doesn’t compile as either C or C++. The Python code has syntax errors (] ending function call Foo). The code below works. The main issue after fixing syntax and compiler errors was declaring the function __stdcall so windll could be used in the Python code. The other option is to use __cdecl (normally the default) and use cdll instead of windll in the Python code.

mydll.c (cl /W4 /LD mydll.c)


#include <stdlib.h>

__declspec(dllexport) void __stdcall Foo(unsigned char** ppMem, int* pSize)
{
    char i;
    *pSize = 4;
    *ppMem = malloc(*pSize);
    for(i = 0; i < *pSize; i++)
        (*ppMem)[i] = i;
}

demo.py (Python 2/3 and 32-/64-bit compatible)

from __future__ import print_function
from ctypes import *

Foo = WinDLL('./mydll').Foo
Foo.argtypes = POINTER(POINTER(c_ubyte)), POINTER(c_int)
Foo.restype = None

mem = POINTER(c_ubyte)()
size = c_int()
Foo(byref(mem),byref(size))
print(size.value, mem[0], mem[1], mem[2], mem[3])

Output

4 0 1 2 3

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